In an external resonator laser emitting apparatus, a resonator structure is formed between a laser light source and an external mirror. As external resonators, a Littrow external resonator and a Littman external resonator are known, in each of which a grating (diffraction grating) is used as the external mirror.
In the Littrow external resonator and the Littman external resonator, light emitted from the laser light source is reflected by the grating and resonated between the grating and the laser light source, so that the resonator structure is formed. Note that light incident on the grating is diffracted at different angles according to wavelength depending on a structure of the grating. Here, examples of structures of the grating with regard to diffraction conditions of incident light include intervals between grooves on a reflection surface (grating intervals), a disposition angle with respect to the laser light source (incident angle of the incident light to the grating), and the like. Therefore, by adjusting such a structure of the grating with regard to diffraction conditions and controlling diffraction angles of light of a desired wavelength band, the wavelength band of light to be returned to the laser light source can be selected. In this manner, in the external resonator laser emitting apparatus using the grating, the wavelength band of light to be resonated can be controlled.
On the other hand, in the external resonator laser emitting apparatus using the grating, when the wavelength of light to be resonated is changed by changing the structure of the grating, in accordance with the change in wavelength, the direction of an optical axis of output light from the external resonator laser emitting apparatus is also changed. Therefore, in the external resonator laser emitting apparatus using the grating, a technique to correct the change in optical axis in accordance with wavelength selection has been proposed.
For example, Patent Document 1 discloses a technique in which the optical axis does not shift in accordance with the change in wavelength by changing a thickness of a volume type diffraction grating (width of incident direction of emitted light from the laser light source to the volume type diffraction grating). In addition, for example, Patent Documents 2 and 3 disclose a technique in which a degree of freedom of selecting the wavelength is increased and the shift of the optical axis is corrected by further providing a prism between the laser light source and the grating or in a latter part of the grating.